The invention concerns a method of transmitting data from a transmitter to one or more receivers, said data being transmitted in the form of a row of characters. These may be received, stored and displayed in the receiver or in each receiver, and the transmitted characters constitute positions in a database. The positions are divided into lines so that the positions in each line are sequenced from the left to the right.
Public paging system are well-known and have found widespread use. The operation of these systems is that information is broad cast from a transmitter by means of radio waves, and that this information may be received by a large number of pagers. Each pager is provided with its own individual address, and when it is desired to transmit information from the transmitter to a specific pager, a combination of the address of said pager and the information concerned is transmitted. Only the pager having the proper address is then capable of receiving the information. Thus, all pagers currently monitor whether their address is transmitted, and when a given pager receives its address, it is activated and receives the subsequent information. This information may then be stored in the pager and be indicated for the person concerned.
In a simple embodiment, the pager merely amits a sound or light signal pointing out to the person that someone wants to get in contact with the person. In another embodiment, numeric information is transmitted to the pager. This typically a telephone number which the person concerned is requested to call. There are also pagers which are capable of receiving alphanumeric information so that short messages may be transmitted to the person carrying the pager.
It is also known to transmit the same information to several pagers at the same time. This is used e.g. when it is desired to transmit the contents of databases to a plurality of receivers. Examples are flight departures, price lists, sports scores or financial information, such as e. g. stock lists. In this case, a large number of pagers are provided with the same address so that the information broadcast to this address will be received by all these pagers. The pagers concerned are normally also provided with an individual address, as described above, so that individual information may be received as well. U.S. Pat. No. 5,241,305 discloses e.g. a system in which information is transmitted simultaneously to several receivers in this manner. This system is unique in that the address which is common to several receivers may be transmitted from the transmitter. Thus, it is possible at a given time to authorize a plurality of pagers to receive a subsequent items of information. Each pager is thus capable of storing a plurality of addresses which may be changed currently at the transmitter side. This provides a very flexible system, in which e.g. a given receiver way be deleted from the database if the person concerned has not paid for the ability to receive the information concerned, and new receivers may correspondingly be added in a simple manner when they take out a subscription for the database concerned.
In connection with other forms of transmission of data, it is well-known to compress these data in order to increase the capacity of the transmission channel in this manner. However, some transmission systems have a very "rigid" protocol which has previously made it difficult to compress data which are transmitted via these channels.
This is the case e.g. for the mentioned paging systems. Here, a protocol called POCSAG (post office code standard agreement) is typically used. A proposal for compression of data which are transmitted to a plurality of pagers by means of this protocol, is known from the above-mentioned U.S. Pat. No. 5,241,305. The method disclosed by this document is based on the fact that in many databases, such as e.g. sports scores, a large amount of the information of the database will be unchanged for a long period of time, while other items of information are changed frequently. Therefore, the positions of the database are divided into two groups, one group for data which are rarely changed, and another group for data which are frequently changed. When the database is transmitted to a receiver for the first time, both groups are transmitted, but in subsequent updates of the database, e.g. when new sports scores are available, it is just necessary to transmit the group of data in which the changes occur most frequently. However, this division into two groups is permanent, and when e.g. a single character is to be changed, it is thus still necessary to transmit all the positions arranged in the group in which changes may frequently occur. Although this method thus provides a certain compression, the method is thus far from optimum, since a considerable amount of superfluous information is still transmitted.
A similar system is known from European patent application EP-A-0 404 007. This system also suffers from the mentioned drawbacks.
It is also known to compress data in another connection by omitting redundant information. Thus, U.S. Pat. No. 4,682,150 discloses a method wherein data to be stored in a storage device are compressed by just storing data that vary between each individual data record. However, this general principle is not directly applicable in connection with transmission of data by means of the above-mentioned POCSAC protocol.
Another method for compression for data in databases to be stored is known from U.S. Pat. No. 4,701,744. Here, a control character is used for switching between two character sets. The drawback of this method is that switching can only be between two specific sets of characters. Thus, the function of the control character is merely to switch from one set of characters to the other, and it does not contribute to the actual compression. Finally, International Patent Application No. WO 92/10035 discloses a datacommunications system in which data prior to transmission are converted into a suitable number system, thereby improving the utilization of the characters available. Here a number system with base 85 is used, because this is precisely the number of options provided by the protocol concerned. Similarly, the POCSAG protocol provides the possibility of using a number system with base 87. However, both of these number systems with base 85 and 87, respectively, are inconvenient in normal use, since they require relatively great allocations of CPU time for decoding on the receiver.
Finally, the European patent application EP 536 831 discloses a corresponding transmission system capable of changing between two transmission modes by means of a control character. In one mode, transmission takes place entirely according to the POCSAG protocol, while in the other mode the address of a text string already stored in a ROM storage in the receiver may be transmitted. Here, too, the control character is thus just used to change between two positions.